BIOL 1030 - Topic 8: Plant Diversity

Plants are a part of …

Archaeplastida

Form + 2 main classes

Form:

• unicellular

• Multicellular

• Colonial

2 main classes:

• Green algae: paraphyletic

• Red algae: monophyletic, have holdfast (root), stipe (stem) and blade (leaf)

Photosynthetic pigments

• allow for adaptation to different environments

• Can be used to identify members

• Different types allow us to use different wavelengths

Plants evolved from green algae, so many plant traits appear in algae, some of which were acquired independently…

• multicellulaeiry

• Photosynthetic pigments

• Cell walls composed of cellulose

Charophytes + traits

• Supergroup archaeplastida, Green algae, and closest relative to terrestrial plants

1. Rings of cellulose synthesizing proteins

distinct circular rings of cellulose-synthesizing proteins in the plasma membrane

2. Flagellated sperm

• In plant species that have flagellated sperm, they most resemble the sperm in charophytes compared to most algae

3. Formation of phragmoplast

• microtubules and associated proteins present in charophytes and land plants after cytokinesis, guiding the assembly of cell plate formation

Molecular analyses of nuclear, mitochondrial and chloroplast DNA also support this

Sproropollenin

• a polymer that prevents desiccation (drying out)

• A waxy, thick molecule that surrounds the outside (outside of cell wall) and allows plant to stay out of H2O for longer

  • common in charophytes algae living st the edge of shallow ponds

Plants colonized land ~450 million years ago, and it had its pros and cons… what were they?

Pros:

• more unfiltered sunlight

• More carbon dioxide

• Nutrients in soil

Cons:

• water is scarce, harder to disperse sperm

• Must grow against gravity

Plants have a wide range of adaptations to allow for survival on land, what are the three main adaptations?

1. Alternation of generations life cycle

• multicellular haploid + diploid stages

2. Walled spores

• contains sporopollenin

3. Apical meristems

• increases exposure to environmental resources

What are some additional derived traits that allowed plants to move on land

• Accessory pigments: help with UV protection, antioxidant effects; like sunscreen! Can also make plants taste bad/poisonous to deter snails

• Cuticle: waxy layer on the outer epidermis to help prevent desiccation

• Stomata (pores): allow exchange of carbon dioxide and oxygen

  • can close in periods of hot, dry conditions to prevent water loss

• Formed symbiotic relationships with fungi

Steps of Alternations of generations

1. Gametophyte produces haploid gametes (1n) via mitosis

2. Two gametes (1n) come together via fertilization to produce a diploid zygote (2n)

3. The zygote (2n) develops into the multicellular sporophyte

4. The sporophyte (2n) produces unicellular haploid spores (1n) by meiosis

5. The spores (1n) develop into multicellular haploid gametophytes

(Broadly) classify plants based on the presence of vascular tissues

• Non-vascular plants (bryophytes)

  • liverworts

  • Mosses

  • Hornworts

• Vascular plants

  • Seedless Vascular Plants

    • Lycophyta (club mosses, spike mosses, quillworts)

    • Monilophytes (ferns, horsetails, whisk ferns)

  • Seed plants

    • Gymnosperms (conifers, ginkgos, cycads, gnetophytes)

    • Angiosperms (flowering plants)

Phylogenetic analysis suggest bryophytes diverged early in plant lineage evolution

What are sporophytes and gametophytes?

Sporophytes and gametophytes are the two alternating, multicellular generations in the plant lifecycle. The sporophyte (2n) is diploid and produces spores via meiosis, which develop into the gametophyte (n).

Nonvascular plants (bryophyte) gametophytes

• Protonema

• Gametophore

• Rhizoids

• Gametangia

  • Archegonia

  • Antheridia

Protonema

• “first thread”

• Threadlike filaments that develop into buds which grow via a meristem to produce the Gametophores

Gametophore

• “ gamete carrier” / “gamete bearer”

• Produces gametes via mitosis

Rhizoids

“Root like” filaments that anchor the Gametphyte

Gametangia

• produces the gametes

  • Archegonia: female, produces the egg

  • Antheridia: male, produces sperm

Bryophyte sporophyte

• seta

• Foot

• Capsule → sporangium

• Peristome

Seta

stalk supporting the sporangium

Foot

• absorbs nutrients from the gametophyte , supports the seta ( attachment point between sporophyte + gametophyte)

Capsule

• sporangium → spore container

• Produces spores via meiosis

Peristome

• “teeth” on the capsule to regulate spore dispersal

What stage is dominant in Bryophytes?

Haploid

What stage is reduced in Bryophytes?

Diploid (2n)

Are Bryophytes dioecious or monoecious?

dioecious - two houses

male and female gametophytes produced separately

Outline the bryophyte Alternation of Generations

1. Spores (1n) develop into threadlike protonema

2. Protonema (n) produces “buds” that develop into gametophytes

• Gametangia: gamete container

3. Sperm must swim through water

• gametes are produced via mitosis

4. Zygote develops into sporophyte embryo (2n)

5. Sporophyte grows a seta that emerges from the archegonium

6. Attached by the foot, the sporophyte remains nutritionally dependent on the female gametophyte (n)

7. Meiosis occurs in the 2n capsule, producing spores

Why are bryophytes ecologically important?

• Help retain moisture

  • Common in moist forests and wetlands, moist bare soil

• Can survive harsh environments (cold, dry)

• Can tolerate complete water loss and can re-establish when moisture returns

  • Poikilohydric: ability to survive extreme dehydration by entering a dormant, "air-dried" state and rapidly reviving when water is available.

• Some Sphagnum moss comprise peat

  • Preserves organisms from decaying due to low temp, pH, o2 levels

  • Important fuel source

  • Horticultural nutrient to retain moisture in soils

  • Carbon sink

Seedless vascular plants

• First to grow tall due to the rigid structure of vascular tissues

• Have additional derived traits

  • Branched sporophytes now dominant stage

  • Not nutritionally dependent on thee gametophyte

  • Some still lack true roots

  • Vascular tissues

  • Still need water for fertilization

What does Xylem do? What is it composed of?

• water transport, structural support

• composed of tracheids and vessel elements, strengthened by lignin

What does Phloem do?

• nutrient transport → transport photosynthates

• composed of sieve tube elements and companion cells

• allows plants to grow tall and still get nutrients to leaves

• allows plants to outcompete shorter plants for light

How did sporophytes and gametophytes evolve to what we see in larger, present day plants?

• They evolved to be of similar size, eventually leading to a larger sporophyte and reduced gametophyte

What stage is dominant in the Fern Life Cycle

Diploid (2n)

Outline the Fern Life Cycle: Alternation of Generations

1. Sporangia release spores (n): Spores develop into a bisexual (monoecious) photosynthetic gametophyte

2. Each gametophyte (n) produces antheridia and archegonia via mitosis. Sperm usually fertilizes the egg off another gametophyte (needs water!)

3. Sperm uses flagella to swim to the egg, attracted via positive chemotaxis to the archegonium

4. The zygote (2n) develops into a new sporophyte (2n) which grows out of the archegonium of the parent gametophyte

5. Sporophytes (2n) produce sori, clusters of sporangia which will produce spores via meiosis

Evolution of roots

• vascular tissues are also found in roots

• roots absorbed water and nutrients from the soil, anchors the plant

“-Phyll” meaning

Leaf

Evolution of leaves (mega vs micro)

• Leaves increase surface area for photosynthesis

  • Megaphyll → larger, more evolutionarily advanced

  • Micropyll → smaller, supported by a single strand of vascular tissue

Shoot system

• comprised of above the ground organs of the plant

  • leaves, stems

Root system

• comprised of the below ground organs

  • roots

What do modified leaves allow for?

Reproduction

What are sporophylls?

Modified leaves that bear sporangia (structures that produce spores).

What do fern sporophylls produce?

: Sori (clusters of sporangia).

What do lycophytes (club mosses + allies) produce?

Strobili (cone-like structures made of sporophylls).

What are strobili?

Small cone-like reproductive structures made of sporophylls.

What are the modified sporophylls in angiosperms?

Carpels and stamens.

What does “homosporous” mean?

Producing only ONE type of spore.

How many types of sporangia do homosporous plants have?

One type.

What do homosporous spores develop into?

A bisexual gametophyte (produces both eggs and sperm).

Example of a homosporous plant?

Ferns

What does the life cycle look like in homosporous plants?

Sporangium → one type of spore → bisexual gametophyte → eggs + sperm

What does “heterosporous” mean?

Producing TWO different types of spores.

What are the two types of spores?

Megaspores and microspores.

How many types of sporangia do heterosporous plants have?

Two types.

Two types of sporangia produce two kinds of pores…

• Megasporophylls → megasporangia

• Microsporophylls → microsporangia

• develop into single sex gametophytes

What adaptations have enabled seed plants to make up the vast majority of plant biodiversity

• Reduced male and female gametophytes

• Pollen

• Ovules

• Seeds

Reduced gametophytes in seed plants

• Microscopic

• Tiny gametophytes can develop directly in the sporangium

  • Protection from the elements such ass UV radiation, dessication

• Gametophytes nutritionally dependent on the sporophyte now

What is the rule among seed plants and what does that entail

Heterospory - two kinds of spores

Megasproropylls - female spore leaves, 2n

Microsporophylls - male spore leaves, 2n

Diagram the process from Mega/Microsporophylls to their respective gametophytes